I posted a WTB ad here because I think my proportioning valve is dead. I'm sending off all 4 calipers to be rebuilt because the pistons are rusty, as the car has been sitting for years. I "tested" the valve by removing the rear calipers and pressing the brake pedal several times. By this point, I had lost a lot of the fluid (through screwing with getting the fronts off, then back on to remove the pistons, etc.) and obviously introduced lots of air into the system. When I pressed the brake pedal, no fluid came out of the rear lines. I can get compressed air through the valve no problem. Is this because I couldn't build enough pressue to get fluid through the valve, or is it actually bad?

I know I should have posted this question before my WTB ad, but I got trigger happy on buying my brake components

You can get brake pressure gauges that thread into the bleeder valves.

Then you could increase pedal pressure and watch both gauges.
At some point ( ~540 PSI) the rears would begin to raise more slowly than the fronts.

So the pressures would track together up to 540, then would diverge above a certain pressure (front's pressure will rise quicker above 540)

Well without the rest of the system, I'm not aware of any other way other than to bench test it. But I do know when I step on mine and put some ass into it, I can hear the proportion limiter "clink" as the valve opens, diverting pressure back to the front brakes. This is done with the car off, sitting still. I'm positive it's working.

Your "test" was meaningless. You can only test it in a properly bled system. all it does is prevent rear lock up on wet roads. Get the calipers fixed the try the system.

That's not quite how it works. What you're hearing is a spring being compressed and released. Its inside a thin sheetmetal cover so its easy to hear.
The actual valve is tiny and made of plastic so I doubt you would hear it.
All that happens is: above a (adjustable) threshhold some of the additional pressure to the rear brakes goes into compressing a big spring, slowing the pressure increase to the rear brakes. It doesn't have any effect on front braking pressure.

Contrary to popular belief, the factory limiting valve doesn't need to be bled separately. It doesn't have a chamber which can hold air.

George, you should get that thing out of your race car and install an adjustable proportioning valve in the cockpit. Then go out and play in a controlled environment until you can lock up the fronts a little before rear lockup occurs.

.... and the learning continues. Thanks Chris - I knew that you know far more about the valve than just about anyone since you did all that extensive testing with them. I was unaware that what I was hearing wasn't the valve itself.

I'll PM or call you for ideas re: the adj prop valve. It's something I wanted to do for awhile, but the plumbing is what confuses me

As always, .